This invention relates to electrodes or membrane electrode assemblies for a solid polymer fuel cell (SPFC; also called “polymer electrolyte fuel cell” or PEFC), and more particularly to an electrode capable of achieving a stabilized power-generation performance even under conditions of unstable humidity in reactant gases supplied to solid polymer fuel cells, and preventing deterioration derived from delayed supply of the reactant gases.
The solid polymer fuel cell has been attracting widespread attention in recent years as being a power source for electric vehicles, and the like. The solid polymer fuel cell can generate electric power at ordinary (sufficiently low) temperatures, and is thus finding various practical applications.
The fuel cell includes an anode and a cathode. The anode is a fuel-gas terminal to which a fuel gas containing hydrogen is supplied. The cathode is an oxidant-gas terminal to which oxidant gas containing oxygen is supplied A chemical reaction then takes place between oxygen in the cathode and hydrogen in the anode, thereby generating electricity. For example, when air is supplied as the oxidant gas to the cathode, chemical energy is converted into electric energy to be supplied to an external load as expressed by the following equations:At the anode: 2H2→4H++4e−At the cathode: O2+4H++4e−→2H2OOverall; 2H2+O2→2H2O  (1)
There is a solid polymer membrane (electrolyte membrane) between the anode and the cathode of the fuel cell. Protons generated during reaction in the anode pass through the solid polymer membrane, and travel with moister to the cathode. Electrons generated during the same reaction in the anode are carried through an external circuit to the cathode. The protons and electrons as thus put together in the cathode react with oxygen in the air to make water.
In the solid polymer membrane fuel cell (also called “proton exchange membrane fuel cell” or PEMFC), moisture should be supplied to constantly maintain proton conductivity of the solid polymer membrane (electrolyte membrane), and thus the reactant gases to be supplied to the fuel cell are humidified in advance.
In general, the solid polymer fuel cell has a layered structure as shown in FIG. 6 in which a single cell 100 is schematically illustrated. Onto both sides of a solid polymer membrane 101 are provided electrode catalyst layers 102a, 102b, and on the outsides thereof are provided gas diffusion layers 103a, 103b, to form a membrane electrode assembly (MEA). On both sides of the MEA are then provided separator plates 104a, 104b, which not only serve to separate each cell but also serve as is manifolds to distribute reactant gases such as fuel gases and oxidant gases between and within the cells. The single cell 100 is formed by sandwiching the above layers between the separator plates 104a, 104b and holdings the layered structure from outside the separator plates 104a, 104b. The electrode catalyst layer 102a or 102b and the gas diffusion layer 103a or 103b make up an electrode (anode or cathode).
It is assumed that a shortage of fuel gases encountered during the process as represented by Equation (1) above would cause corrosion of carbon in the gas diffusion layers 103a, 103b, as expressed by Equation (2) as follows:2H2O+C→4H++CO2⇑  (2)
If corrosion proceeded as above, catalyst supporting carbon black would be consumed; this would disadvantageously deteriorate the membrane electrode assembly, and eventually deteriorate the fuel cell itself.
Several attempts have been made to prevent such corrosion of carbon from proceeding and to eliminate the resulting disadvantages, mostly with consideration given to the reaction as in Equation (1); for example, Applicant previously has devised an approach of giving water-retaining capability to the electrode catalyst layers (see JP 2003-168442 A). Another approach disclosed in WO 01/15254 A is to add a catalyst for accelerating electrolysis of water to the electrode catalyst layers.
The above existing approaches of giving water-retaining capability or adding a catalyst for accelerating electrolysis of water to the electrode catalyst layers would be effective for a transient shortage of fuel gases, but repeatedly encountered shortages of fuel gases (e.g., due to abrupt acceleration, or the like) which would be assumed in actual driving situations, or rated driving conditions, would disadvantageously result in flooding due to the enhanced water-retaining capability in the electrode catalyst layers. The flooding is a phenomenon in which water is retained in gas diffusion channels such as pores formed in the electrode catalyst layers and inhibits diffusion of gases. Flooding would not only lower the performance of the membrane electrode assembly but also inhibit supply of fuel gases under operating conditions of the fuel cell such that shortages of fuel gases are repeatedly encountered, and would expand a region in which fuel gases are insufficient in the anode, so that corrosion of carbon could proceed, thus decreasing the performance of the membrane electrode assembly.
In order to avoid causing power generation performance of the fuel cell to lower due to flooding within the cell, a pore-making material may be added to form a porous structure of the electrode catalyst layers 102a, 102b which serves to remove water in the cell (see JP 8-180879 A). As the gas diffusion layers 103a, 103b, a porous material having a current-collecting property may be provided on the outsides of the electrode catalyst layers 102a, 102b; for example, carbon paper having a porosity of 80%, etc. may be employed.
The pore-making material added to the electrode catalyst layers 102a, 102b would indeed improve the power generation performance under high-humidity conditions where a plenty of water exists in the cell 100 because the pore-making material in the electrode catalyst layers 102a, 102b would facilitate drainage of water from the electrode catalyst layers 102a, 102b, thus serving to prevent flooding; however, under low-humidity conditions, only adding the pore-making material to the electrode catalyst layers 102a, 102b would rather lead to disadvantageous effects of lowering the power generation performance because water required to maintain proton conductivity of the solid polymer membrane 101 would be drained out through pores formed by adding the pore-making material.
The present invention has been made to address the above-described disadvantages.